Depending on the size of the group, it can be hard to engage with them all at once without it turning into a shouting match. However, there are a few ways to get some good feedback:
Talk to the thought leaders in the group. Someone (Or a few someones) is probably organizing and running these events, or is at least a very active participant. This is a good person to know, and while you should be careful about it being a single data point, the thoughts of this person will be extremely useful.
Show up and listen. Most people at user groups go to talk about what they use the product for and to seek advice on things that are hard to do. Make some notes and you can pretty quickly get a feel for what they do with the application and what the major sticking points are.
Ask them to engage one-on-one. If you have surveys, forums, or other existing ways to gather user feedback make the group aware of it and invite them to participate there as well.
All of these ideas really depend on what sort of things you want feedback on, but this is a great resource and you're lucky to have it.